1. Field of the Invention
This invention relates generally to improvements in the stability characteristics of switched voltage regulators, and more particularly, to improvements in the stability characteristics of a pulse width modulated voltage regulator-converter/power converter having cascaded LC--LC filter networks.
2. Description of the Prior Art
In the recent past, pulse width modulated voltage regulator--converter/power converter systems have found wide use in military and commercial equipment due mainly to high performance and reliability. However, there are some problems encountered with prior art systems, especially when the systems are required to operate with a range of bulk voltages and under varying complex static load conditions, such as in high performance computer systems. A basic requirement of any voltage regulator-converter/power converter system is to maintain load regulation to the input switching regulator, sometimes termed "pre-regulator," by providing feedback from the pre-regulated output to the switching regulator or in the alternative, providing feedback from the output load to the pre-regulator.
FIG. 1 depicts a prior art voltage regulator--converter/power converter showing the connections for both types of feedback aforementioned. Usually, only one type of feedback is utilized in any one power supply although it is feasible, but with penalties in cost, to use both. Generally, the system is comprised of a regulator-converter section 10, an averaging filter section 12, a power converter section 14, a power converter filter section 16 and a pulse width modulated/clock/logic system control 18. In addition, for purposes of maintaining load regulation, a type A feedback network 20 or a type B feedback network 22 is provided. Regulator--converter section 10 includes a regulator converter interstage transformer 24 and a regulator converter power transistor 26. Averaging filter 12 includes a flyback diode D.sub.1, an inductor L.sub.1 and a shunt capacitor C.sub.1 connected to ground. Also, power converter 14 includes a power converter interstage transformer 28, first and second power converter transistors 30 and 32 and power converter output transformer 34. Power converter filter 16 includes a pair of rectification diodes D.sub.2 and D.sub.3 connected in a full rectification configuration, an inductor L.sub.2 and a shunt capacitor C.sub.2 connected to ground. Load impedance Z.sub.0 is representative of the various loads that the system of FIG. 1 can drive.
Considering first a system utilizing type A feedback, pulse width modulated/clock/logic system control 18 generates switching pulses which turn regulator converter power transistor 26 on and off at the frequency rate generated. Accordingly, a voltage with an amplitude substantially equal to the bulk voltage V.sub.1 and at the frequency of operation is filtered by averaging filter 12. The values of inductor L.sub.1 and shunt capacitor C.sub.1, generally, are chosen so that there is some voltage fluctuation or ripple voltage in the regulated voltage V.sub.2. In addition, flyback diode D.sub.1 ensures that voltage transients due to the inductance of inductor L.sub.1 are clamped to ground when regulator converter power transistor 26 is off. A portion of regulated voltage V.sub.2 is connected in type A feedback network 20 and applied to pulse width modulated/clock/logic system control 18 modulating the switching pulses generated therein in accordance with the variations in regulated voltage V.sub.2. Switching pulses are also generated from pulse width modulated/clock/logic system control 18 driving first and second power converter transistors 30 and 32 via power converter interstage transformer 28 in an alternating fashion. Regulated voltage V.sub.2, in addition to being applied to type A feedback network 20, is also applied to the center tap of power converter output transformer 34. Finally, the switched voltage at primary windings N.sub.1 and N.sub.2 of power converter output transformer 34 is stepped down and coupled to the secondary windings N.sub.3 and N.sub.4 thereof. The power converter stage 14 is connected in a push-pull configuration, accordingly, the secondary voltage is full wave rectified by rectification diodes D.sub.2 and D.sub.3 and filtered by inductor L.sub.2 and capacitor C.sub.2 providing an output voltage V.sub.3.
Briefly summarizing, in a system utilizing type A feedback, regulated voltage V.sub.2 is sampled by type A feedback network 20 and applied to associated circuitry, aforementioned, maintaining thereby, a fixed voltage, i.e., V.sub.2, to power converter 14. Power converter output transformer 34 steps down the aforementioned fixed voltage providing, after filtering by power converter filter 16, a regulated output voltage V.sub.3. The principle disadvantage of a system utilizing type A feedback is the poor load regulation (typically plus or minus 10 percent due mainly to the feedback voltage at the input to type A feedback network 20 being isolated from the output load Z.sub.0. However, systems utilizing type A feedback do provide a stable form of regulation since the inductor-capacitor pole pair, i.e., L.sub.1 -C.sub.1, is located in the left-half S-plane.
Still referring to FIG. 1, type B feedback network 22 represents a prior art attempt to improve the poor regulation associated with utilizing type A feedback network 20 aforementioned. Type B feedback network 22, as depicted, closes the loop from the output of the power converter filter 16 to pulse width modulated/clock/logic system control system 18 including, now, within the loop, inter alia, averaging filter 12 and power converter filter 16. This connection does improve load regulation; however, stability problems are generally encountered because of the cascaded filter networks, L.sub.1 C.sub.1 -L.sub.2 C.sub.2. In addition, the system becomes less stable as system gain is increased since the inductor-capacitor pole pair, i.e., L.sub.1 -C.sub.1 tends to migrate into the right-half S-plane.
There have been attempts to solve the type B feedback stability problem including increasing the value of C.sub.1 of averaging filter 12 to attenuate the feedback gain and achieve zero gain versus frequency crossover before phase inversion occurs at the resonant frequency of inductor L.sub.2, and capacitor C.sub.2 of power converter filter 16. This technique has generally been successful. Nevertheless, the penalties incurred, that is, large inductor-capacitor (L.sub.1 -C.sub.1) and/or high output noise levels limit the utilization of the aforementioned solution in applications where high performance is of paramount importance, such as computer systems, for example. In addition, load impedance Z.sub.0, generally contains additional decoupling capacitors which tend to lower the resonant frequency of L.sub.2 -C.sub.2 of power converter filter 16 causing, thereby, phase inversion to occur at a lower frequency, and in the process, compromising phase stability and gain margin. It has also been found, in practice, that a system utilizing type B feedback has limited application with dynamic loading because of fourth order filter effects which create output ripple voltages in excess of specified limits for high performance systems.
An alternate technique to perform the regulator-converter function that would eliminate the foregoing stability problems can be envisioned by reference to FIG. 1. In this system, a separate pulse width modulated/clock/logic system control 18, for example, is added to drive power converter 14. Type A feedback network 20 is connected as shown providing a closed feedback loop to provide line regulation, i.e., regulation of bulk voltage V.sub.1, and an additional type B feedback network connected from the output of power converter filter 16 to the newly added pulse width modulated/clock/logic system control to provide power converter regulation for load variations. Notwithstanding the foregoing system being a solution to the stability problem, the principle disadvantages are the additional cost and size of the control system and bias supplies need to provide the control and bias functions.